1. Field of the Invention
The present invention is directed to a polymeric binder system and method of making, a method of forming thin-walled articles by extrusion, and to articles produced thereby. More particularly, the invention relates to a polymeric binder system containing an organic solvent to lower the binder viscosity and form a uniform extrudable mixture at near-ambient temperatures.
2. Background of the Invention
Techniques are known for producing thick-walled articles by extrusion utilizing ceramic and metal powders which are combined with organic chemicals that function as binders. Such methods typically utilize polymeric binders, such as polyvinyl butyral (for example, Butvar(copyright) B-76, Solutia, Inc., St. Louis, Mo.), which are burned out by a combination of pyrolysis and oxidation to decompose the binder to gases. Such extrusion binders tend to melt before reacting and decomposing to gases, and are suitable for forming thick-walled articles in which the binder melting inside a three-dimensional porous green body provides capillarity that pulls the mass together. However, such binders are not satisfactory for forming thin-walled articles such as thin-walled electrolyte tubes. This is because the articles become distorted as the capillarity from the melted binder reduces the wall thickness as the sheet of (powder-filled) liquid becomes unstable in shape and breaks apart into droplets, analogous to the droplet formation from a thin stream of water dripping from a faucet. Even without droplet formation, the thin-walled shapes can become distorted by gravity acting on the green ceramics held together with the liquefied binder at the early stages of binder burn-out.
One partial solution to this ceramic processing problem has been to add materials such as a block copolymer of styrene/butadiene as part of a thermoplastic elastomer binder system. Such materials can be formed by traditional rubber-making techniques utilizing shear in two-roll mills. For example, a method of making thin-walled electrolyte tubes is described in commonly assigned U.S. Pat. No. 4,615,851, in which a thermoplastic elastomer is mixed with ceramic powder on hot rolls which are heated in excess of 200xc2x0 C. The extrudable powder mixture prepared by this method allows thin-walled shapes to be formed, burned out, and sintered with little deformation of the thin walls, because the block copolymer remains a high viscosity liquid after melting until decomposition.
Because of the inconvenience of the high temperature rubber-making process to make the extrudable mass, there has been continued interest in the development of improved polymeric binder systems for ceramic and metal powders for injection molding and extrusion. For example, a polyacetal binder is used in Catamold(copyright) Ultraform ceramic-filled polyacetal injection molding polymeric binder system (BASF Corporation, Wyandotte, Mich.). Such a binder provides an advantage for burn-out of both thin- and thick-walled structures due to its nearly isothermal catalytic decomposition of the solid binder to gaseous phases. However, in the acetal process, the gases used in the catalyzed solid-to-gas volatilization reaction and the resulting gases are both toxic and flammable.
Yet another known binder system and process is referred to as xe2x80x9cgelcasting.xe2x80x9d This process utilizes chemical monomers which are solidified by cross-linking immediately after intermixing with an activator in the moments just prior to injection into a closed mold. Although this binder system allows both thin and thick sections to be formed one-by-one in closed molds, it is not practical for use in high-volume, continuous shear forming operations such as extrusion of thin-walled tubular articles due to the short time that passes before the binder becomes like a solid. Furthermore, the monomer and activator are toxic. The binder solidification reaction in gelcasting differs from that of the thermoplastic elastomer system in that it is irreversible. This leads to further losses on processing and more constraints in scheduling of sequential processing steps.
Accordingly, there is still a need in the art for an improved polymeric binder system and method for forming extruded thin-walled articles that can be sintered with little deformation of the thin walls.
The present invention meets that need by providing a polymeric binder system and method for extruding thin-walled articles which does not require heat for preparation of the extrudable mixture. The polymeric binder components are dissolved with shear in an organic solvent, such as, for example, toluene or tetrahydrofuran, to form a liquid polymeric binder system which is then mixed with ceramic or metal powders. After removal of substantially all of the organic solvent by evaporation, the resulting extrudable mass may be extruded to form thin-walled tubes, which, upon firing through burn-out and sintering, retain their thin-walled form. Thus, the extrudable mixture of the polymeric binder system and powder provides an easy means to make thin-walled extruded articles that retain their shape upon subsequent ceramic processing.
According to one aspect of the present invention, a method of forming an extruded thin-walled article is provided comprising providing a polymeric binder system of a substantially homogeneous solution of a polymeric binder and an organic solvent. A ceramic or metal powder is then added to the polymeric binder system to form a slurry mixture. Preferably, the ceramic powder comprises yttria-stabilized zirconia. The metal precursor powder preferably comprises nickel oxide plus yttria-stabilized zirconia.
After addition of the ceramic or metal powder, the organic solvent is evaporated from the mixture, and the remaining mixture is extruded from a die to form a thin-walled article. The mixture is preferably extruded at a temperature of between about 100 to 135xc2x0 C., and more preferably, about 120xc2x0 C.
The method preferably further includes subsequent heating of the extruded thin-walled article to burn off the binder and to sinter the article.
By xe2x80x9cthin-walledxe2x80x9d article, it is meant an article having a wall thickness of less than about 2 mm, and preferably less than about 0.5 mm. For fuel cell applications, we have achieved extruded wall thicknesses as thin as 0.2 mm.
The polymeric binder system of the present invention includes a polymeric binder and an organic solvent, which together are subsequently used to intermix and bind together the ceramic or metal powder into a shaped form. The polymeric binder preferably comprises a thermoplastic block copolymer, a first thermoplastic polymer and a second thermoplastic polymer which is different from the first thermoplastic polymer, and a plasticizer. The thermoplastic block copolymer preferably comprises a copolymer of styrene and butadiene.
The first thermoplastic polymer preferably comprises polystyrene and the second thermoplastic polymer preferably comprises polyindene.
The plasticizer included in the polymeric binder preferably comprises at least one oil and at least one wax. The oils and waxes allow a wide range of melting temperatures within the polymeric binder system. Furthermore, the lower melting components provide porosity that is useful for the ease of subsequent burn-out of the thermoplastic block copolymers and the first and second thermoplastic polymers.
The polymeric binder may further include a processing aid such as an antioxidant to adjust thermal decomposition rates and to reduce any gas-blocking layer formation during burn-out.
The organic solvent in the polymeric binder system is preferably toluene or tetrahydrofuran. The organic solvent may also be selected from cyclohexane, methylcyclohexane, benzene, ethylbenzene, styrene, lower chlorinated aliphatic hydrocarbons, tetrahydrofurfuryl alcohol, phenol/acetone, dimethyltetrahydrofuran, dioxane, methyl ethyl ketone, diisopropylketone, cyclohexanone, ethyl acetate, butyl acetate, n-butyl phthalate, carbon disulfide, and tributyl phosphate.
The polymeric binder system of the present invention is preferably prepared by dissolving the polymeric binder components in the organic solvent to form a substantially homogeneous liquid. The components of the polymeric binder system are preferably mixed in the following order: thermoplastic block copolymer, organic solvent, (and optional antioxidant), followed by the addition of the remaining components in any order. Preferably, shear processing is applied to insure good intermixing of the polymeric components.
The polymeric binder system and method of the present invention provide an improvement over prior binder systems in that it is simpler to make the binder-powder mixture because the organic solvent allows the preparation of the polymeric binder system at lower temperatures of about 50xc2x0 C. Solvent removal by evaporation also occurs at a lower temperature of about 50xc2x0 C. (or other moderate temperature, including room temperature). In addition, there is increased reproducibility in the extruded articles over prior art rubber-compounding methods because the components in the binder (other than some of the organic solvent) are not lost by evaporation or burning off of some fractions of the polymeric binder system during the compounding process, a problem which occurs with prior art systems that require heating to 200xc2x0 C. during manufacture of an extrudable material.
Accordingly, it is a feature of the present invention to provide a polymeric binder system and method for extruding thin-walled articles which does not require heating to high temperatures (greater than about 100xc2x0 C.) to achieve mixing of the extrudable mass and which results in a thin-walled green ceramic article which retains its shape during and after burn-off and sintering. Other features and advantages of the invention will be apparent from the following description and the appended claims.